Chantelle Price, 3rd all-time HS 800 runner is running XC and may win state (PA, not a small state) in this, her senior year. She has pretty good 400 speed (she goes out sub-60 for the first 400).
Maybe your coach thinks
a) your performance at 400 does not match your 200 and you are definitely not a 'speed' guy of the 100m variety, so endurance is lacking;
and/or
b) you have never run distance of ANY sort before and who knows what you can do in the 800 unless you try it (in addition, they still run the 600 indoors and he might want to see how you can do there). He may have found that good athletes usually go 'short' because they are the glamor events and pushing the up side is useful. In HS our star 200/400/LJ guy ran XC for conditioning in the fall. This was way back in the early- mid-60s and he ran Junior-year marks of 48.4/440, 22 220, was best in the league in the 180 hurdles, and 23'6" in the LJ. He ran the 880 the first race of the track season (February/L.A. area) and ran 1:57.0 on a dirt track despite doing the first lap in 54.0 seconds
Finally, if you are just a sprinter there are possibly some restrictions on fall training but doing XC eases those constraints. It also makes it more feasible to recover between races when doing 400 and 4x400 (and maybe a 200) in the same meet. It might also improve your running efficiency and provide more secondary muscle strength which can help avoid sprinting injuries.